v.checker: fix postfix expr on generic types (#9887)

vlib/v/checker/checker.v

@@ -5843,7 +5843,7 @@ fn (c &Checker) has_return(stmts []ast.Stmt) ?bool {
 }
 
 pub fn (mut c Checker) postfix_expr(mut node ast.PostfixExpr) ast.Type {
-	typ := c.expr(node.expr)
+	typ := c.unwrap_generic(c.expr(node.expr))
 	typ_sym := c.table.get_type_symbol(typ)
 	is_non_void_pointer := (typ.is_ptr() || typ.is_pointer()) && typ_sym.kind != .voidptr
 	if !c.inside_unsafe && is_non_void_pointer && !node.expr.is_auto_deref_var() {

vlib/v/tests/generics_test.v

@@ -33,6 +33,49 @@ fn test_infix_expr() {
 	assert plus<string>('a', 'b') == 'ab'
 }
 
+fn plus_one<T>(a T) T {
+	mut b := a
+	b++
+	return b
+}
+
+fn minus_one<T>(a T) T {
+	mut b := a
+	b--
+	return b
+}
+
+fn test_postfix_expr() {
+	assert plus_one(-1) == 0
+	assert plus_one(byte(0)) == 1
+	assert plus_one(u16(1)) == 2
+	assert plus_one(u32(2)) == 3
+	assert plus_one(u64(3)) == 4
+	assert plus_one(i8(-10)) == -9
+	assert plus_one(i16(-9)) == -8
+	assert plus_one(int(-8)) == -7
+	assert plus_one(i64(-7)) == -6
+	assert minus_one(0) == -1
+	assert minus_one(byte(1)) == 0
+	assert minus_one(u16(2)) == 1
+	assert minus_one(u32(3)) == 2
+	assert minus_one(u64(4)) == 3
+	assert minus_one(i8(-8)) == -9
+	assert minus_one(i16(-7)) == -8
+	assert minus_one(int(-6)) == -7
+	assert minus_one(i64(-5)) == -6
+	// the point is to see if it compiles, more than if the result
+	// is correct, so 1e-6 isn't necessarily the right value to do this
+	// but it's not important
+	delta := 1e-6
+	assert plus_one(1.1) - 2.1 < delta
+	assert plus_one(f32(2.2)) - 3.2 < delta
+	assert plus_one(f64(3.3)) - 4.3 < delta
+	assert minus_one(1.1) - 0.1 < delta
+	assert minus_one(f32(2.2)) - 1.2 < delta
+	assert minus_one(f64(3.3)) - 2.3 < delta
+}
+
 fn sum<T>(l []T) T {
 	mut r := T(0)
 	for e in l {